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A '''function''' is a rule that maps one set of values to another set of values, assigning to each value in the first set exactly one value in the second.  For instance, one function may map 1 to 1, 2 to 4, 3 to 9, 4 to 16, and so on.  This function has the rule that it takes its input value, and squares it to get an output value.  One can call this function <math>f</math>.  

==Rigorous Definition==
Let <math>A</math>,<math>B</math> be [[set]]s and let <math>f\subset A\times B</math>, where <math>A\times B</math> is the [[Cartesian product]] of <math>A</math> and <math>B</math>.  (That is, <math>f</math> is a [[relation]] between <math>A</math> and <math>B</math>.)  We say that <math>f</math> is a ''function from <math>A</math> to <math>B</math>'' if and only if 

* For every <math>a \in A</math> there is some <math>b \in B</math> such that <math>(a, b) \in f</math>, and
* if <math>(a,b)\in f</math> and <math>(a,c)\in f</math> then <math>b=c</math>.  (Here <math>(a,b)</math> is an [[ordered pair]].)

==Introductory Topics==
===Domain and Range===
The domain of a function is the [[set]] of input values for the argument of a function.  The range of a function is the [[set]] of output values for that function.  For an example, consider the function: <math>f(x) = \sqrt{x^2-9}</math>.  The domain of the function is the set <math>{x:|x|>3}</math>, where <math>x</math> is a real number.  The range is the set of all non-negative real numbers because the square root can never return a negative value.

===Real Functions===
A real function is a function whose [[range]] is in the real numbers. Usually we speak about function whose domain is also a [[subset]] of the real numbers.

===Graphs===
Functions are often graphed. To find out if a graph is a function, it must stand up to the [[vertical line test]].

===Inverses===
The inverse of a function is a function that "undoes" a function.  For an example, consider the function: f(x)<math> = x^2 + 6</math>.  The function <math>g(x) = \sqrt{x-6}</math> has the property that <math>f(g(x)) = x</math>.  In this case, <math>g</math> is called the '''(right) inverse function'''.  (Similarly, a function <math>g</math> so that <math>g(f(x))=x</math> is called the '''left inverse function'''. Typically the right and left inverses coincide on a suitable domain, and in this case we simply call the right and left inverse function the '''inverse function'''.) Often the inverse of a function <math>f</math> is denoted by <math>f^{-1}</math>.

==Intermediate Topics==
===Injections, Surjections, Bijections===
*An [[injection]] (or one-to-one function) is a function which has distinct values for distinct arguments within a given domain.

By definition, <math>f:A\to B</math> is injective if <math>f(a)=f(b) \Rightarrow a=b </math>, or equivalently, <math>a\neq b \Rightarrow f(a)\neq f(b).</math>  

Examples:

* <math>f(x) = x</math> is injective from <math>\mathbb{C} \rightarrow \mathbb{C}.</math>
* <math>f(x) = x^2</math> is injective from <math>\mathbb{R^+} \rightarrow \mathbb{R}.</math>
* <math>f(x) = x^2</math> is not injective from <math>\mathbb{R} \rightarrow \mathbb{R}.</math>

Injectivity of a function <math>f: A \rightarrow B</math> implies that <math>f</math> has an inverse.  Furthermore, if <math>A</math> and <math>B</math> are finite sets, injectivity implies <math>|A|\leq |B|</math>.  


*A [[surjection]] (or onto function) maps at least one element from its domain, <math>A,</math> onto every element of its range, <math>B.</math>

Examples:

* <math>f(x) = x</math> is surjective from <math>\mathbb{C} \rightarrow \mathbb{C}.</math>
* <math>f(x) = x^2</math> is surjective from <math>\mathbb{R} \rightarrow \mathbb{R^+}</math>
* <math>f(x) = x^2</math> is not surjective from <math>\mathbb{R} \rightarrow \mathbb{R}.</math>


*A [[bijection]] (both one-to-one and onto) is a function, <math>f: A \rightarrow B</math> that is both injective and surjective.  

If <math>f</math> is an injection from <math>A \rightarrow B</math> and <math>g</math> is an injection from <math>B \rightarrow A,</math> then there exists a bijection, <math>h,</math> between <math>A</math> and <math>B</math> by the [[Schroder-Bernstein Theorem]].

===Monotonic functions===
A function <math>f:A\to B</math> is called [[monotonically increasing]] if <math>f(x_1)\geq f(x_2) </math> holds whenever <math>x_1>x_2 </math>. If the inequality holds strictly <math>(f(x_1)>f(x_2)) </math>,
then the function is called [[strictly increasing]].

Similarly, a function <math>f:A\to B</math> is called [[monotonically decreasing]] if <math>f(x_1)\geq f(x_2) </math> holds whenever <math>x_1<x_2 </math>. If the inequality holds strictly <math>(f(x_1)>f(x_2)) </math>,
then the function is called [[strictly decreasing]].

==Advanced Topics==
===Functions of Real Variables===
A real function is a function whose [[range]] is in the real numbers. Usually we speak about function whose domain is also a [[subset]] of the real numbers.

===Continuity===
Intuitively, a continuous function has the propriety that its graph can be drawn without taking the pencil off the paper. But the reality about continuous functions is more complex. 

====Epsilon-Delta Definition====
A function <math>f:E\to\mathbb{R}</math> is called continuous at <math> x_{0} </math> if, for all <math> \varepsilon >0 </math>, there exists <math>\delta >0</math> such that <math> |x-x_0|<\delta </math> and <math>x\in E \Rightarrow |f(x)-f(x_0)|<\varepsilon </math>.

====Heine Definition====
The previous definition of continuity at <math> x_{0} </math> is equivalent with the following: for every sequence <math> (x_n)_{n\geq 0} </math> such that <math>\lim_{n\to\infty}x_n=x_0 </math>, we have that <math> \lim_{n\to\infty}f(x_n)=f(x_0) </math>.

It is easy to see that a function is continuous in [[isolated point]]s, and is continuous in [[accumulation point]]s [[iff]] the limit of the function in those points equals the value of the function.

A function is continuous on a set if it is continuous in every point of the set.

====Properties of Continuous Functions====
* The sum and product of two continuous functions are continuous functions.
* The composition of two continuous functions is a continuous function.
* In any closed interval <math>[a, b]</math>, there exist real numbers <math>c</math> and <math>d</math> such that <math>f</math> has a [[maximum]] value at <math>c</math> and <math>f</math> has a [[minimum]] value at <math>d</math>.
* Intermediate Value Theorem (see below)

=====Intermediate value theorem=====
If a function is continuous, then it has the [[Intermediate Value Theorem]]. The converse is not always true.
''Proof'':...

===Differentiability===
Differentiability is a smoothness condition on functions.  For functions of one variable, differentiability is simply the question of whether or not a [[derivative]] exists.  For functions of more than one variable, the notion of differentiability is significantly more complicated.  In the case of both one and multivariable functions, differentiability implies continuity.

A single-variable function <math>f(x)</math> is differentiable at <math>x=a</math> iff <math>\lim_{x\rightarrow a} \frac{f(x)-f(a)}{x-a} \in \mathbb{R}</math>.  The derivative is the value of this [[limit]].

===Integrability===
All continuous functions are integrable, as well as many non-continuous functions.

===Convexity===
A twice-differentiable function <math>f(x) </math> is concave up (or ''convex'') in the interval <math>[a,b] </math> iff <math>f''(x)>0</math> in the interval <math>[a,b] </math> and concave down (or ''concave'') iff <math>f''(x)<0 </math>. The points of inflection, when the concavity switches, of the function occur at the roots of <math>f''(x) </math>.

==Notation==
A common notation to define <math>f</math> is: <math>f(x) = x^2</math> (where the <math>x^2</math>, of course, is merely an example).  This tells us that <math>f</math> is a function that squares its argument (its input value). Note that this "rule" can be arbitrarily complicated and doesn't need to be given by a simple formula or description. The only requirement is that <math>f(x)</math> should be uniquely determined by <math>x</math>. The following are examples of functions:

*<math>f(x)=x ^ {2}+2x-2</math>

*<math>f(x)=\sin(\log{x})</math>

*<math>f(x)=x^2</math> for <math>x>0</math>, otherwise <math>f(x)= \sin{x}</math>

*<math>f(x)=p(g(x))</math>

*<math>g(x)=F'(x)</math>

== History of Functions ==
Without being used explicitly, the notion of function first appears with the ancient Greeks and Egyptians. 

The rigorous definition was stated in the 19th century and is the result of the works of some famous mathematicians: [[A.L. Cauchy]], [[Leonhard Euler]], and [[Bernhard Riemann]]. With the development of [[set theory]], a new branch of mathematics appeared, [[mathematical analysis]], in which the notion of function has a central role.

The current notation used is attributed to [[Leonhard Euler]].

==Problems==
===Introductory===
*Define <math>x\otimes y=x^3-y</math>. What is <math>h\otimes (h\otimes h)</math>?

<math> \mathrm{(A) \ } -h\qquad \mathrm{(B) \ } 0\qquad \mathrm{(C) \ } h\qquad \mathrm{(D) \ } 2h\qquad \mathrm{(E) \ } h^3 </math>
([[2006 AMC 10A Problems/Problem 2|Source]])
===Intermediate===
*The [[function]] f is defined on the [[set]] of [[integer]]s and satisfies
<math>
f(n)=
\begin{cases}
 n-3 & \mbox{if }n\ge 1000 \\
 f(f(n+5)) & \mbox{if }n<1000
\end{cases}
</math>
Find <math>f(84)</math>.
([[1984 AIME Problems/Problem 7|Source]])

===Olympiad===
*Let <math>f</math> be a function with the following properties:
# <math>f(n)</math> is defined for every positive integer <math>n</math>;
# <math>f(n)</math> is an integer;
# <math>f(2)=2</math>;
# <math>f(mn)=f(m)f(n)</math> for all <math>m</math> and <math>n</math>;
# <math>f(m)>f(n)</math> whenever <math>m>n</math>.

Prove that <math>f(n)=n</math>.
([[1969 Canadian MO Problems/Problem 8|Source]])

===Advanced===
*Describe all polynomials <math>P(x)</math> such that <math>P(x + 1) - 1 = P(x) + P'(x)</math> for all <math>x</math>. 

(<url>weblog_entry.php?t=182628 Source</url>)

==See also==
* [[Algebra]]
** [[Functional equation]]
** [[Polynomials]]
* [[Calculus]]
** [[Limit]]
** [[Derivative]]
** [[Integral]]

[[Category:Algebra]]
[[Category:Definition]]
[[Category:Functions]]